Conventionally, in heavy duty pneumatic tires used on trucks, buses, and the like, a rib-based tread pattern is used in which, for example, a tread portion is provided with four main grooves extending in the tire circumferential direction, and five rows of ribs, partitioned by the main grooves and extending in the tire circumferential direction (see Japanese Unexamined Patent Application Publication No. 2008-1293A and International Patent Publication No. WO/2010/147076). In such rib-based tread patterns having four main grooves, typically, the center rib is disposed at the position of the tire equatorial plane.
In contrast, in recent years, wide-base pneumatic tires having an increased development width of the tread portion have been proposed, but in cases where a rib-based tread pattern such as that described above is applied as-is, the width of each of the ribs increases and the rigidity thereof increases. Consequentially, such pneumatic tires are prone to railway wear and similar uneven wear. Additionally, in wide-base pneumatic tires, tire contact pressure across the ribs easily becomes nonuniform, and this type of nonuniform tire contact pressure both promotes uneven wear and is a cause of cracks and/or fractures forming in the ribs.